The crank-gear of a bicycle usually includes two cranks each associated with a pedal and mounted at the respective extremities of a shaft which traverses axially a crank-gear case provided in the lower part of the frame. This shaft likewise bears the toothed sprocket plate or plates intended to drive the chain. The assembly rotates around the axis defined by the crank-gear case. The cranks being diametrally aligned in a rigid manner, it is found that at their top and bottom dead center points, i.e. when they are vertical, the effort provided by the cyclist is not transformed into a driving force for the chain while at some angular distance on either side of these dead center points such effort is inefficiently transmitted.
In an effort to overcome these difficulties, it has already been proposed (for example by the French utility certificate 2.526.392) to disconnect these cranks from the sprocket plate and to have them turn about an axis different from that about which the sprocket plate rotates. Transmission means are then provided to connect the cranks to the sprocket plate in a manner such that the offset between the two rotating systems is at no time bothersome. There results however that the angular position of the two cranks relative to one another is in constant variation during rotation of the crank-gear.
From the disclosure in the French utility certificate previously mentioned, it may be determined that the transmission means are formed from two guides, one per crank, radially oriented and fixed to these cranks. Each guide cooperates with a roller which is fixedly mounted in the sprocket plate in a manner to describe with the latter an orbital movement around the center of the sprocket plate.
One of these guides is directly placed on a crank (that on the right for instance), while the other is placed on a lever fixed to the axis of the crank-gear and united with the other crank (arranged at the left on this axis).
Furthermore, the abovementioned utility certificate provides a mechanism regulating the offset between the axes of the cranks, on the one hand, and of the sprocket plate on the other hand, in order to be able to adjust the relative inclination of the cranks to one another.
If this prior art mechanism thereby permits effectively to avoid vertical aligmnent of the cranks and thus their simultaneous passage through the dead center top and bottom points, it is to be noted that such result is obtained by means of a construction of an impractical nature which renders it almost unemployable.
Initially in effect the transversal space requirement of this crank-gear is considerable and the cranks are for this reason spread apart from one another by a substantial and impractical distance. This is not only unfavourable from an aerodymamic viewpoint, but also and above all creates considerable discomfort for the cyclist who must thus pedal with much too great a spread between his feet.
This substantial transversal space requirement along the axis of the cranks is due to the fact that the shaft of the crank-gear must project from the right of the case over a considerable distance in order to be able to form a rotation bearing for the right-hand crank as well as means for fastening the lever which assures transmission of the force to the sprocket plate from the lefthand crank. In other words, the shaft of the crank-gear exhibits a substantial overhang out of the crankgear case while precisely, the entire effort of the cranks is applied to this overhanging shaft end. As the stress to which the shaft is thus subjected varies constantly, said shaft is subjected to variable and repetitive strains which give rise to a very substantial risk of breakage.
Furthermore, since at least one of the cranks includes a guide, it cannot be of a type normally available in commerce. This increases not only the cost of the crank-gear, but likewise the cost of repairs should such crank have to be replaced for one reason or another.
In addition, the mechanism described in this prior art document is completely exposed during its function to dirt and to the impact of gravel, including the slides and the rollers which cooperate together for transmission of the movement, in a manner such that the mechanism becomes rapidly unusable if not regularly and carefully cleaned.
The purpose of the invention is to provide a crankgear of the general type mentioned hereinabove, but arranged to avoid the cited difficulties, such crank-gear being of a compact construction practically fluid tight and thus not subject to breakdowns due to dirt and of a solidity comparable to that of standard crank-gears which include a rigid coupling between the cranks and the sprocket plate.
It should also be noted that a crank-gear exhibiting an offset between the rotation axes of the sprocket plate and the crank has been described in the French utility certificate No. 2,520.692. The construction of this mechanism is vey similar to that of the arrangement described in the French utility certificate 2.526.392 already mentioned and thus includes the same difficulties. Furthermore, in this second utility certificate the relative arrangement of the sprocket plate and the cranks is such that the latter are aligned in their vertical position. This known construction thus maintains the disadvantages of the rigid crank-gears as far as the simultaneous passage of the cranks through the top and bottom dead center points is concerned.